1. Field of the Invention
The present invention relates to a wavelength-division multiplexing (WDM) optical transmission system, in which pluralities of wavelength bands are multiplexed and transmitted bidirectionally along a pair of optical fibers.
2. Description of the Related Art
In recent years, the amplifier repeater transmission systems using the WDM optical signals have been widely used in optical fiber transmission systems. This is because the WDM optical signals, which comprise pluralities of multiplexed wavelength bands (channels), can be amplified in a simple single operation by optical amplifier repeaters using optical fiber amplifiers.
In particular, systems for transmitting high-density WDM optical signals over long distances use a method for reducing the effects of four-wave mixing (FWM: also referred as "four-photon mixing"), which is caused by the third-order nonlinearity in the optical fiber and combinations of different wavelengths. According to this method, positive-dispersion optical fibers are inserted periodically along transmission optical fibers which cancel the negative accumulated dispersion in the transmission optical fibers, in order to prevent signal distortion caused by accumulated dispersion, while ensuring that the chromatic dispersion of the transmission optical fibers is not zero within the signal wavelength bands. As a result, the accumulated value of the total dispersion is approximately zero, and there is no local accumulation of zero dispersion.
However, chromatic dispersion of optical fibers has wavelength dependence on higher-order dispersion. Consequently, accumulated wavelength dispersion at the central wavelength of the multiplexed wavelength bands may for instance be as shown in FIG. 1. In FIG. 1, in comparison with the central wavelength .lambda..sub.b, which has a one segment dispersion D.sub.c, the longer (maximum) wavelength .lambda..sub.a, suffers positive accumulated dispersion of D.sub.y (ps/nm) giving dispersion D.sub.y -D.sub.c, while the shorter (minimum) wavelength D.sub.x (ps/nm) accumulates negatively, with dispersion of -D.sub.x -D.sub.c.
FIG. 2 is a block diagram illustrating a diagrammatic configuration for compensating accumulated dispersion in a conventional WDM optical transmission system. The conventional WDM optical transmission system has a plurality of optical transmitters 1, a multiplexer(MUX) 5 combining (multiplexing) a plurality of different wavelength bands transmitted by the optical transmitters 1. The optical transmitters include the optical transmitters 1 which transmit optical signals at minimum, central and maximum wavelength bands. And the conventional WDM optical transmission system further has a demultiplexer(DEMUX) 23, a plurality of optical receiver 27 and a plurality of DCOFs 31,32,33. The DCOF 31 has the amount of compensating dispersion D.sub.x +D.sub.c, the DCOF 32 has the amount of compensating dispersion D.sub.c and the DCOF 33 has the amount of compensating dispersion -D.sub.y +D.sub.c. The two-way optical fiber cable shown in FIG. 2 has a pair of optical fibers, and the optical fiber cable comprises a plurality of segments. In FIG. 2, a plurality of in-line amplifiers (optical amplifier repeaters) 9 are also shown as periodically inserted in the optical fiber cable.
As FIG. 2 shows, in order to achieve overall accumulated dispersion of zero, the signals at respective wavelength bands are first separated by means of a demultiplexer 23. Then, dispersion compensation DCOFs 31,32,33 having compensating dispersions which are counter to the accumulated dispersions are inserted, the length of the inserted DCOFs being selected as approximately sufficient to cancel the one segment dispersion D.sub.c. In this way, it is possible to provide a dispersion compensator to compensate different accumulated dispersions as required.
For instance, when WDM signals are transmitted within a 1.55 .mu.m wavelength band, accumulated dispersion resulting from higher-order dispersion and wavelength dependence of fiber material is negative for wavelength bands shorter than central wavelength .lambda..sub.b, where accumulated dispersion is zero. Therefore, since an optical fiber having zero dispersion within a wavelength band of 1.3 .mu.m has positive dispersion of approximately 17 ps/km-nm in a wavelength band of 1.55 .mu.m, such an optical fiber has been used as the DCOF to compensate negative dispersion.
By contrast, for wavelength bands longer than the central wavelength .lambda..sub.b, accumulated dispersion resulting from the wavelength dependence of fiber material and higher-order dispersion is positive. Therefore, an optical fiber which has negative dispersion within a wavelength band of 1.55 .mu.m has been used as a DCOF for compensating positive dispersion.
More specifically, for example, when oceanic long-haul optical submarine cables using WDM transmission have the above configuration, the DCOFs inevitably become extremely long as the number of multiplexed wavelength bands increases.
For instance, in WDM transmission with 20 wavelength bands being propagated over 9000 km at wavelength intervals of 0.6 nm, when higher-order dispersion is 0.1ps/km-nm.sup.2, the signal with the minimum wavelength would require nearly 500 km of 1.3 .mu.m zero-dispersion DCOF, even after compensating the transmission line dispersion to the central wavelength.
FIG. 3 is a block diagram illustrating another configuration for compensating accumulated dispersion in a conventional WDM optical transmission system. As FIG. 3 shows, a DCOF 34 corresponding to one segment dispersion D.sub.c is inserted in the transmission line on the receiver side of the transmission cable.
However, as FIG. 4 shows, in an actual communications system the optical transmission line is comprised from two-way optical fibers, along which optical signals are transmitted bidirectionally. In other words, the optical cable contains a pair of optical fibers. Then, in the system that a DCOF 34 corresponding to one segment dispersion compensation is inserted at the receiver side of the transmission cable (i.e. the pair of optical fibers) as described above, when transmitting in the opposite direction, a dispersion compensation cable must be connected at the transmitter side, leading to a loss of bidirectional symmetry.
Such asymmetrical dispersion accumulation is particularly likely to cause disparity in transmission characteristics between the two transmission directions in long-haul systems. Therefore, in conventional systems, the receiver unit contains DCOFs 35 having a length sufficient for compensating the final dispersion compensation segment of the transmission line. As a consequence, dispersion compensation at the receiver unit has had to be carried out with respect to the sum of the amount of compensation of chromatic dispersion, caused by wavelength difference and higher-order wavelength dispersion, and the amount of dispersion compensated in one dispersion compensation segment of the transmission line. This has a disadvantage that the DCOF for equalizing the dispersion, which is provided to the receiver, must be extremely long.
Furthermore, as FIG. 5 shows, another method has been proposed in which elongation of the dispersion equalization optical fibers is eased. In FIG. 5, DCOFs 31 on the receiver side in FIG. 2 were respectively divided into two parts 38 and 37. And the parts 38 having dispersion (1-m)D.sub.x are inserted into the transmitter side, and the part 37 having dispersion mD.sub.x +D.sub.c are inserted into the receiver side. In this method, to compensate for the dispersion deviation caused by the wavelength difference with the higher-order wavelength dispersion, DCOFs 38 having an approximately 50% (m=0.5) of the dispersion compensation against the dispersion deviation at each wavelength are inserted at the transmitter side. FIG. 6 depicts the accumulated wavelength dispersion at the central wavelength at this point. As FIG. 6 shows, the configuration depicted in FIG. 5 improves the transmission characteristics.
However, even when the configuration illustrated in FIG. 5 is implemented in an actual lightwave communications system transmitting bidirectionally along optical fibers, the receiving unit must contain DCOFs 37 for one dispersion compensation segment of the transmission line at the receiver side, in order to maintain transmission line symmetry.
According to the conventional optical transmission system configuration described above, the receiver unit must contain a DCOF 37 for the last dispersion compensation segment of the transmission line. Moreover, each wavelength band requires a DCOF to compensate the residual accumulated dispersion of the wavelength bands.